The Granite Controversy: Neptunism VS Plutonism

The dominant role of Neptunism, denominated after the Roman god of the sea, in geology during the 18th and 19th century can be traced back to the significance of the Mining Academy in Freiberg (Saxony) and especially the teachings of professor of mineralogy Abraham Gottlob Werner (1749-1817).

Fig.1. Frontispiece of "The Granite Controversy" by H. H. Read (1957), drawn by D. A. Walton. This cartoon is fitting to the 19th century controversy between Neptunism and Plutonism, however it was drawn to deal with the problem arose in the 20th century if granites are mainly of metamorphic origin.

His research and lectures introduced a classification-scheme of rocks based on the sequence of layers and age, rather than by the types of minerals as had been previously done. Werner was not the first to propose water as origin of the rocks, but he was the most consequent supporter and divulgator of this theory.
In his book of 1787 "Kurze Klassifikation und Beschreibung der verschiedenen Gesteinsarten." (Short classification and description of the various rock-formations) he also speculated on the formation of earth. Early earth, born by the agglutination of cosmic matter, originally consisted of a primordial ocean with various dissolved elements, rocks were then deposited by crystallisation and sedimentation from this former universal ocean in a very specific order: first the oldest and hardest rocks - granites, gneiss and schist, then basalts and finally limestone and other common sedimentary rocks. After the last rocks crystallized, the sea-level dropped and erosion started to form the last, recent sedimentary rocks.

Johann Wolfgang Goethe, philosopher, poet and "Neptunism-geologist", summarizes the argument as follows:

"All observations agree, as much of them were carried out recently, that granite is the lowermost kind of mountain on our earth, that all the rest are found on or beside it, it itself in contrast is not overlay by nothing else, so it, even if it not compose the entire earth, its nevertheless compose the lowermost crust that is accessible to us." Neptunism seemed well established by the observations in the field. In 1788 Werner publishes his work "Bekanntmachungen einer von ihm am Scherbenberger Hügel über die Entstehung des Basalts gemachte Entdeckung" (Notes about a discovery at the hill of Scherbenberg about the formation of basalt), where he describes the transition of sand, argillaceous deposits and conglomerate to basalt. The apparent horizontal bedding of the basaltic plateaus and under- and overlying sediments in central Europe was considered by Werner as a proof of the sedimentary origin of these rocks, unfortunately he never visited and observed active volcanoes.

A contrasting theory was proposed by English naturalist James Hutton: primordial rocks are the results of magmatic intrusions and eruptions. This theory was named after the Roman god of the underworld Plutonism. The resulting conflict divided geologist and an intense research started, hoping to solve the riddle of rock formation.

In 1806 the Italian mining engineer Count Giuseppe Marzari-Pencati studied the geology of Tyrol.

Fig.2. Count Giuseppe Marzari-Pencati.

Fig.3. Magmatic dikes ("Serpentinite") cutting trough marbles ("modified limestone"), the last also in contact with a magmatic intrusion of "granite" - as seen by a geologist in 1848 (figure from Geo-Mineralogische Skizzen über einige Täler Tirols).

Fig.4. And magmatic dikes of the Ladinian (228-237Ma) cutting trough marbles (former reef limestone), as seen at the locality of Mountain of Dos Capel, Dolomites, near Predazzo.

Based on his studies of the flank of the mountain of Canzoccoli, at the village of Predazzo in the Fassa Valley (Dolomites), he recognized the superimposition of granitic rocks onto sedimentary marbles - a geological impossibility according to Neptunism - granite, the oldest rock, should be found always below any other geological formation. In 1820 Pencati publishes his observations in the journal "Nuovo Osservatore Veneziano.", his article shocked geologists....